Centrifugal separation of suspensions of solid particles in liquids is essential in many procedures. For example, centrifugal separation is used to purify protein conjugates and to desalt and purify peptides prior to electrophoresis. There are many other centrifugal separations that are used in industry and in laboratories.
Usually a centrifugal separation is effected by placing the suspension to be separated in a small plastic tube, called a centrifuge tube, which is placed in a cavity in a centrifugal separating apparatus, and then the tube and its contents are subjected to a spinning action so that centrifugal force causes many multiples of gravity within the tube. As a result, the solids in the suspension accumulate at the bottom of the tube in what is know as a pellet. The pellet is recovered by decanting the liquid. The tube must be removed from the centrifuge for it to be examined to determine if the extent of the separation is adequate. If the initial centrifugal separation is not complete, a second separation is necessary for a longer period of time or at a higher rotational speed.
In some centrifugal separations the pellet must be dried which is frequently accomplished by decanting the liquid from the centrifuge tube and then filling the tube with a drying medium such as alcohol, which dissolves the water in the pellet that was not removed by decanting. The drying process may dislodge some of the particles from the pellet so that after drying it is necessary to subject the tube containing the drying medium to further centrifugal separation to force all of the solid particles back into the pellet. Any time the centrifuge tube is removed from the centrifuge cavity and replaced in a different orientation with respect to a centrifuge cavity, the pellet is disturbed and the time for centrifugal separation must be repeated. Reforming the pellet is time consuming and it is particularly wasteful if the objective of removing the centrifuge tube was only for the purpose of observing whether the separation was complete.